1. Technical Field
The present invention relates to a semiconductor device and a method of manufacturing the same.
2. Related Art
In development of advanced CMOS (complementary MOS) devices having transistors being more and more shrunk in size, degradation of drive current due to depletion of polysilicon electrodes has been noticed as a problem. Techniques for avoiding depletion of the electrodes, through adoption of a metal gate electrode, has therefore been discussed. Pure metals, metal nitrides and silicides have been investigated as materials for composing the metal gate electrode. In any case, threshold voltage (Vth) of each of N-type MOSFET and P-type MOSFET needs be adjustable to an appropriate level.
For example, the advanced CMOS transistor needs Vth adjusted to ±0.1 V or around, so that it is necessary to use a material having an effective work function (EWF) equivalent to the work function of N-type polysilicon (4.0 eV) for the N-type MOSFET, and to use a material having an EWF equivalent to the work function of N-type polysilicon (5.2 eV) for the P-type MOSFET.
At present, titanium nitride (TiN) has widely been investigated as a candidate material for the metal gate electrode, appreciating is thermal stability and readiness of patterning into gate electrode geometry. TiN placed on a high-k gate insulating film is, however, known to show an EWF at around the middle of the band gap of Si, and is therefore incapable of achieving low Vth by this technique alone.
Kunihiko Iwamoto et al. describe a technique in Applied Physics Letters 92, 132907, 2008 “Experimental evidence for the flatband voltage shift of high-k metal-oxide-semiconductor devices due to the dipole formation at the high-k/SiO2 interface”, that EWF may be increased and thereby Vth may be lowered through shifting of the flatband voltage (VFB) towards the positive bias side, by inserting an aluminum oxide film (capping film) between the TiN electrode and the high-k gate insulating film.
On the other hand, Japanese Laid-Open Patent Publication No. 2009-239080 describes a technique of controlling height of Schottky barrier formed at the interface between a semiconductor substrate and a metal layer formed on the semiconductor substrate, by forming in between an oxide film mainly composed of Hf or Zr.